1. Field of the Invention
This invention relates to the design of tunable transconductance elements and monolithic continuous-time filters.
2. Background Art
Monolithic, continuous-time, high-frequency filters are preferably based on capacitors and transconductances rather than operational amplifiers. In general, the transconductance building blocks are subject to a number of strict requirements. They must have a large dynamic range. They must be built with simple circuitry so as to reduce parasitic capacitance characteristics at high-frequency operation. They must be easily tunable for use in programmable filters, a common application. They are preferably configured in a fully differential circuit for superior PSRR (power supply rejection ratio), CMRR (common-mode rejection ratio), and second-order distortion cancellation. They preferably operate on 5 volts.
In the past, transconductances have suffered severely in their tunability. Prior art circuits are not suitable for high frequencies and meet the desirable criteria above. High-frequency filters implemented with transconductances, theoretically the most desirable building block for such filters, were unusable because of a bandwidth and/or dynamic range limitation.
Prior art devices encumber the task of implementing a differential input or multiple input transconductance element.
Therefore, it is an object of the present invention to increase the achievable frequency range of operation of a tunable transconductance element with large dynamic range.
It is also an object of the present invention to facilitate the setting of pole frequencies and pole quality factors of a biquad filter section based on these transconductance elements.
It is also an object of the present invention to provide a transconductance circuit element with minimized parasitic capacitance and output conductance.
It is a further object of the present invention to create a transconductance element whose basic design can accommodate single or multiple inputs.
It is also an object of the present invention to design a multiple input transconductance which maintains low output admittance (high output impedance).